1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
In the battery, a shutdown mechanism utilizing a separator, a current breaking element, etc. was used as the means for securing the safety in the case where the battery was over-charged.
In the shutdown mechanism utilizing a separator, the porous film used as the separator is melted when the battery temperature is elevated by the over-charging so as to close the pores of the porous film, thereby breaking the current passageway. On the other hand, when it comes to the current breaking element, the electrical resistance of the current breaking element is increased by the elevation of the battery temperature so as to break the current passageway. The current breaking element of this type includes, for example, a PTC thermistor and a fuse.
In addition to the shutdown mechanism and the current breaking element, a current breaking rupture is employed in a battery for securing the safety in the case where the battery is over-charged. The current breaking rupture is designed such that a rupture is opened by the increase in the inner pressure of the battery so as to prevent the rupture of the battery. Alternatively, the current breaking rupture is designed such that the current passageway is broken by the deformation of, for example, the rupture.
However, the energy of the battery has been increased in recent years so as to make it more and more difficult to secure the safety of the battery when the battery has been over-charged. It should be noted in this connection that, with increase in the energy of the battery, the battery is caused to generate heat rapidly when the battery is over-charged. In addition, the amount of the generated heat is increased. Further, the thermal stability of the battery has been lowered, with the result that the thermal runaway of the battery is brought about under temperatures lower than those in the prior art. In the thermal runaway, it is difficult for the current passageway to be broken by the conventional shutdown mechanism utilizing the separator or the conventional current breaking element, with the result that it is possible for the battery to be ignited or ruptured. Such being the situation, it is difficult to secure the safety of the battery when the battery is over-charged.
What should also be noted is that, in the case of employing the current breaking element or the current breaking rupture, the number of parts required forming the battery is increased so as to increase the cost of the parts and to make complex the manufacturing process of the battery. It follows that the manufacturing cost of the battery is increased.
Incidentally, a secondary battery comprising a separator formed of a porous polyolefin film is disclosed in Japanese Patent Disclosure (Kokai) No. 2000-348706. The separator used in this secondary battery has (a) a thickness of 5 to 50 μm, (b) a porosity of 40 to 80%, (c) a peak pore diameter of 0.05 to 0.2 μm, (d) an electric resistance of the electrolytic solution of 0.3 to 2.5 Ω·cm2/25 μm, (e) a pin-piercing strength of 300 gf/25 μm or more, (f) a tensile strength of 300 kg/cm2 or more, and (g) a thermal shrinking rate at 110° C. of 10% or less. It is taught that the separator for the battery described above permits improving the permeability of the electrolytic solution through the separator and also permits improving the discharge rate characteristics of the secondary battery.
However, a serious problem is left unsolved in the separator for the battery disclosed in the patent document quoted above. In general, an electrode group is manufactured by spirally winding in a flattened shape a laminate structure comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. If the separator disclosed in the patent document quoted above is used for manufacturing the electrode group of the construction described above, a serious problem is generated in the secondary battery comprising the particular electrode group. Specifically, the separator included in the electrode group is thermally shrunk by the elevation of the battery temperature, if the secondary battery is over-charged. Because of the shrinkage of the separator, the positive electrode and the negative electrode are caused to be brought into contact with each other in the peripheral region of the electrode group. In other words, an internal short circuit is brought about within the electrode group so as to give rise to the problem of ignition of the secondary battery.